Currently, an optical fiber used in an optical fiber communication system that is widely used has such a structure that the circumference of one core is surrounded by a cladding, and an optical signal propagates through the core, whereby information is transmitted. With the spread of the optical fiber communication system in recent years, the amount of transmitted information is increasing dramatically. With an increase in the amount of transmitted information, a large number of optical fibers ranging from several tens to several hundreds are used in the optical fiber communication system, to thereby enable large-volume and long-distance optical communication.
In order to decrease the number of optical fibers used in such an optical fiber communication system, a technique of transmitting a plurality of signals via light that propagates through cores of a multicore fiber in which the circumferences of a plurality of cores are surrounded by one cladding is known.
However, in the multicore fiber, inter-core crosstalk may occur. Thus, when the diameter of the multicore fiber decreases, since the inter-core distance decreases, the crosstalk occurs more easily. Thus, there is a demand for a multicore fiber that can reduce inter-core crosstalk.
Patent Document 1 described below discloses an example of a multicore fiber that can reduce such inter-core crosstalk. In this multicore fiber, a plurality of cores are disposed in the cladding, and a leakage reducing portion (low-refractive index layer) having a lower refractive index than the cladding is formed so as to surround the circumferential surface of each of the cores. Specifically, the circumferential surface of each of the cores is surrounded by an inner cladding layer having the same refractive index as the cladding, and the circumferential surface of each of the inner cladding layers is surrounded by the leakage reducing portion that has a lower refractive index than the cladding.